Measuring the refraction of the eye is difficult as the eye is a living organ and is constantly changing and moving. Even with an intelligent and cooperative patient fixating a target, the eye will be moving because of micronystagmus. Without this constant motion, the eye cannot function. It is an established phenomenon that the eye is a differential sensing mechanism; if an image is perfectly fixed on the retina, the brain causes the image to fade from view. Thus, to see effectively, the eye must be constantly moving.
Alignment is a major problem for most methods of refraction. The optical axis of the eye must match the optical axis of the measuring instrument. Several methods of eye alignment have been used in the prior art.
One method to help ensure proper eye alignment is video imaging. Typically such video imaging enlarges the eye many times for display on a monitor so the examiner can determine that the eye is properly fixated and, hopefully, aligned.
A second method uses an eye tracker that follows the movement of the eye. Because of the double-pinhole principle used in most machines, alignment is critical.
In order to avoid the alignment problem, it has been known to use a measurement beam that over-fills the pupil so that alignment becomes less critical. However, a significant disadvantage of such instruments is that long measurement times (up to 20 seconds) are required to measure each eye.
Because the eye is constantly changing, measurements taken at different times can show different values due to random effects. The longer the time interval required for the measurement, the longer the integration of that measurement to previously obtained values. This is required to average out the random effects to improve the signal-to-noise ratio and, thereby, improve accuracy. One way to avoid movement errors is to have a very short measurement time, on the order of one millisecond. Unfortunately, random errors can appear in these short measurements.
The above problems of the measurement of refraction are compounded in the case of children. Objective refraction of children has always been associated with problems. Children have wide powers of accommodation such that conventional testing may obtain varied and inaccurate refractive readings. Further, children simply do not stay in the same place for overlong periods of time. Consequently, a different method of autorefraction is required. Finally, large and imposing optical apparatus--for example most conventional autorefractors--tend to excite and frighten the youthful subjects. This is especially true if the intimate presence of an operator proximate the child patient is required. Simply stated, the excited and frightened juvenile subject falsely accommodates--and the measurement of such refractions can be in error.
Accommodative error is the biggest problem in providing accurate and reproducible measurements. In order to see objects close-up, the lens of the eye must change shape, become "fatter" so that the nearby object will be clearly focused on the retina. Looking into a box, or any type of instrument, even when the object being viewed inside the box is at optical infinity induces accommodation. This is a psychological phenomenon. It has been discovered that when a subject looked through a small hole (such as a hole in a wall so that the subject thought he was looking into another room although the viewed object might be close by) caused accommodation to be relaxed.
Even older children--intelligent and trying to cooperate--because of lack of experience may not be able to readily position themselves in the chin/forehead rests, properly fixate the target, and remain still for the requisite measurement time. For infants and younger children refraction is even more difficult.
Bringing an instrument close to the child's eyes may cause the child to close his eyes and resist examination. In this case, measurement must be taken from a distance. One method of measuring from a distance of about one meter is using photorefraction techniques. Current instrumentation replaces photographic film with CCDs to get quick readouts. Nonetheless, no one has actually "solved" the problem of accommodation.
Three main methods used to relax accommodation (refraction is measured at optical infinity, "making" the eye change its optics to see a target at optical infinity) are the following: 1) having the patient fixate an object 5 meters or further away, 2) fixate a point of light or a "featureless" pattern, and 3) "fog" the eye with a positive lens so that accommodation causes the fixated target to become more blurred, thus encouraging relaxation of the accommodative mechanism.
The most commonly used method to relax accommodation is the fogging method. With a positive lens, the eye is refracted to get an initial reading. Then, an in-focus target such as a sailboat on the ocean, a tractor in a field, or a balloon in the sky, is presented to the patient, which is fogged to relax accommodation and get the patient's refractive reading. However, in cases of latent hyperopia in some children, fogging is not effective and a cycloplegia must be used to relax accommodation.
Another problem in providing accurate and reproducible refractions is that the basic meridional method of refraction requires great accuracy of the initial measurements. It can be mathematically shown that meridional error as little as one-quarter diopter can cause entirely erroneous results.
Meridional refraction requires a minimum of three meridional measurements, and these data are put into Lawrence's formula to calculate mean sphere, cylinder, and axis. If measurements are not accurate, as noted above, or if astigmatism is irregular, refraction can be in significant error. One approach to this problem is to search for the principal axes of astigmatism to provide better accuracy.